The making of expandable particles of styrene-type polymers is well known. It comprises two basic aspects, polymerization of styrene-type monomer and the impregnation of the polymer with a blowing agent. The polymerization reaction is usually conducted either in bulk or in suspension. Where the bulk polymerization method is used to prepare the polymer the impregnation step is usually conducted in a separate reactor. In the case of the suspension polymerization method the impregnation may be carried out either as a step contiguous to the polymerization by introducing the blowing agent to the reactor at a certain point in the conversion of monomer to polymer or at the completion of the polymerization, or as a step completely separate and independent of the polymerization. In the latter case, the bead product of the suspension polymerization is taken from the reactor, washed, and the impregnation step is conducted by re-suspending the polymer in water. The latter method has the advantage that out of the total polymer beads obtained from the suspension polymerization, one can select only those beads having a particle size suitable for expandable styrene-type polymer usage and use the rest for other purposes.
The impregnated particles are usually pre-expanded and aged before molding to stabilize the bead structure and control the final density of the molded article. The molding step is conducted by filling the mold cavity with the pre-expanded polymer particles and heating them above the softening point usually by use of superheated steam. The minimum time period, after the introduction of steam into the mold, that the article must remain in the mold before removal is generally called the "cooling time". The cooling time is often a very substantial portion of the molding cycle and hence the reduction thereof has an important commercial significance for enhancing production efficiency.
Barkhuff et al. U.S. Pat. No. 2,989,782 discloses expandable polystyrene beads which yield molded resin foam articles having high strength at elevated temperatures, the surface shell only of said beads containing a compatible organic compound that is soluble in the polystyrene resin. Various classes of compounds are broadly described as suitable for said "compatible organic compound". Mentioned among them are fatty acids and esters thereof including monoglycerides of 12 to 20 carbon atom fatty acids.
Ingram et al. U.S. Pat. No. 3,389,097 describes a method of making fast-cooling expandable polystyrene beads wherein styrene monomer is polymerized in an aqueous suspension system up to a point between about 60-95% monomer conversion and then a fatty acid bisamide is added to the system and the polymerization completed and the impregnation is separately conducted after the completion of the polymerization.
Stahnecker et al. U.S. Pat. No. 3,461,088 describes a method of making fast-cooling expandable polystyrene beads wherein polystyrene beads which have been or are being pre-expanded are coated with an organic compound having a boiling point of more than 95.degree. C. and a softening point of not more than 120.degree. C. A large number of such organic compounds suitable for the invention are disclosed in the patent including esters of fatty acids and aliphatic hydrocarbons substituted by at least one hydroxy group.
Buchholz et al. U.S. Pat. No. 3,468,820 describes a method of suspension-polymerizing styrene in the presence of a blowing agent and 0.001 to 1% of a water-insoluble high molecular weight polymer of a vinyl ester, vinyl ether, acrylic ester, or methacrylic ester which is soluble in the monomer but is not homogeneously miscible with the polymer obtained. The patent discloses that the resultant polymer has fine and uniform cells and also a relatively short cooling time.
Roberts et al. U.S. Pat. No. 3,480,570 describes a method of coating expandable polystyrene beads with 0.02-0.3% by weight of sorbitan fatty acid ester containing 12-18 carbon atoms in the fatty acid moiety and having an HLB (hydrophilic-lipophilic balance) number of 1.5-9. The patent discloses that the cooling time is reduced typically to 50-80% of the "normal" value.
Ingram et al. U.S. Pat. No. 3,503,908 (British Pat. No. 1,174,749) describes a method of making fast-cooling expandable polystyrene beads wherein the impregnation is conducted in an aqueous suspension system comprising a surface-active agent selected from the group consisting of polyoxyethylene mono-esters of fatty acids, polyoxyethylene sorbitan mono-esters of fatty acids, and polyoxyethylene mono-ethers of fatty alcohols said compounds having an HLB (hydrophilic-lipophilic balance) number of at least about 15.
Heiskel et al. U.S. Pat. No. 2,789,028 describes expandable polystyrene beads having a short cooling time comprising as a surface coating agent 0.05-0.5% by weight, based on the blowing agent containing styrene polymer, of a mixture of mono-, di- and tri-esters of glycerol with saturated fatty acids having 12-22 carbon atoms. As examples of methods of applying the coating agent there are mentioned a method of applying powdery coating agent onto the beads by means of a rotating drum and a method of treating the expandable beads with a solution of the coating agent in a solvent.
Alvares et al. U.S. Pat. No. 3,991,020 discloses an expandable polystyrene composition wherein 0.005-1.0% (based on the weight of polystyrene) of a polyether derived from propylene oxide or from propylene oxide and ethylene oxide is homogeneously dispersed on at least the surface of the expandable particles. The admixture of polyethers and polystyrene is prepared by mixing the expandable polystyrene particles with the additive, or a portion of the additive may be incorporated in the polystyrene during polymerization and the remaining portion added to the subsequent mixture. This patent also discloses a process for preparing expandable polystyrene wherein the monomer is polymerized in aqueous suspension, a blowing agent is admixed with the polymer formed during and/or at the end of the polymerization, and the polymer containing the expanding agent is separated from the suspension medium and subsequently mixed with 0.005-1.0% by weight of polyethers derived from propylene oxide or from propylene and ethylene oxides. The main advantages discloses in this patent are that the expandable beads have an anti-lumping property during the pre-expansion and a short cooling time.
Japan Kokai No. 33958/1977 describes expandable polystyrene beads having a good fusion property wherein 0.2-2 weight % of a fatty acid partial ester of glycerol, glycerol dimer or sorbitan is attached to the surface of the beads, and also a method of making same wherein polystyrene beads which have already been impregnated with a blowing agent are suspended in an aqueous system containing a fatty acid partial ester of glycerol, glycerol dimer or sorbitan and an anionic surfactant, and after the mixture has been stirred for a sufficient length of time the beads are collected.
Japan Kokai No. 64277/1978 describes expandable polystyrene beads having a good fusion property wherein the surface of the polymer beads is coated with a glycerol tri-ester of a higher fatty acid. Said glycerol tri-ester is applied to the surface of the polymer beads which have already been impregnated with a blowing agent.
Japan Kokai No. 125,129/1980 describes a method of making expandable thermoplastic polymer beads wherein, during or subsequent to the impregnation of a blowing agent to thermoplastic polymer beads, a nucleating agent is incorporated into the thermoplastic polymer beads containing the blowing agent at a temperature above the softening point of the beads. Among examples of suitable nucleating agent to be incorporated into the thermoplastic polymer beads impregnated with a blowing agent, there are mentioned esters of glycerol, pentaerythritol, and sorbitan including glycerol mono-stearate, pentaerythritol mono-stearate and sorbitan mono-stearate.
Watts U.S. Pat. No. 4,042,541 discloses expandable polystyrene beads having a reduced cooling time containing, uniformly dispersed within the beads, from about 100 to 300 ppm of stearic acid and from about 40 to 120 ppm of calcium stearate.
Davis et al. U.S. Pat. No. 4,174,427 discloses a method for making expandable polystyrene beads suitable for making drinking cups and other impervious containers wherein the polymerization of styrene is conducted in the presence of polyvinyl pyrrolidone, and, after impregnation, the beads are washed in an aqueous solution of a hydrophilic alkoxylated nonionic surfactant, which is solid at room temperature and has 70-90 weight percent of the polymer chain accounted for by oxyethylene units, for about 3 to 10 hours at a temperature of 40.degree.-55.degree. C. to remove contaminated grafted polyvinyl pyrrolidone from the surface of said beads. The primary purpose of this patented process is to obtain polystyrene beads which are expandable into impervious containers and which are free from mold-filling and static-electricity problems.
Shibata U.S. Pat. No. 4,238,570 describes expandable particles of a styrene polymer having a short cooling time which comprise on the surface thereof, an ester obtainable from an aliphatic carboxylic acid and an aliphatic alcohol, the ester having no hydroxyl group in the molecule and being a solid at room temperature or a mixture of said ester and a finely divided lubricant. As a preferred method of incorporating the ester of an aliphatic carboxylic acid with an aliphatic alcohol onto the surface of the expandable particles of a styrene polymer, mentioned therein is a method in which the ester in a powdery state is mixed mechanically with the expandable polymer beads.
Spicuzza U.S. Pat. No. 4,312,957 describes a process of making fast-cooling expandable polystyrene beads, wherein a synergistic mixture of surfactants, comprising (1) a first surfactant selected from the group consisting of polyoxyethylene sorbitan monoesters of fatty acids, polyoxyethylene monoesters of fatty acids and polyoxyethylene monoethers of long-chain fatty alcohols, and (2) a second surfactant selected from the polyoxyethylene monoethers of alkylphenols, is incorporated into styrene polymer particles during the process of impregnating the particles with a blowing agent.